In the implementation of laser-induced fluorescence (LIF) for the detection of vapor-phase organic compounds that
accompany hazardous materials, multiphoton excitation offers a significant advantage over single photon methods. In
particular, if the absorption spectra of unwanted background molecules overlap that of the target molecule, single photon
LIF is plagued by false positives. Multiphoton methods alleviate this difficulty by requiring that the target molecule be
in resonance with multiple molecular transitions. A promising multiphoton method is stimulated Raman adiabatic
passage (STIRAP). This method involves a counterintuitive sequence of laser pulses which is capable of transferring
100% of the target molecules to the desired excited state from which fluorescence is to be observed.
As a precursor to more complex molecules, we demonstrate the STIRAP technique on sodium vapor using the 3p (2P1/2)
← 3s (2S1/2) and 5s (2S1/2) ← 3p (2P1/2) transitions. This is the first time STIRAP has been achieved on a vapor using
picosecond lasers. We produced light to couple the states using two synchronously pumped OPG/OPAs (pumped by the
355 nm light from a picosecond YAG). We measured the fluorescence from the 5s state to both 3p states (2P1/2, 2P3/2)
and from both 3p states to the 3s state with monochromator using a gated CCD to eliminate Rayleigh scattered light.
Our results indicate a four to five-fold increase in the transfer efficiency to the 5s state when the laser pulse that couples
the 3p and 5s states precedes the laser pulse tuned to the 3p ← 3s transition.